The present invention relates to a method and apparatus for differential thermal analysis. Differential thermal techniques generally consist of applying heat simultaneously to a sample material and a reference material and measuring a parameter, such as differential power input, as the sample goes through a physical or chemical change. In differential thermal analysis (DTA), the sample and reference are heated or cooled according to a programmed rate, and the temperature differential between the sample and reference is measured as the scan proceeds. In differential scanning calorimetry (DSC), differential power rather than differential temperature is measured. The differential power represents the difference in energy required to maintain the sample and reference in accord with a heating or cooling program.
In addition to DSC and DTA, other differential thermal techniques also exist to measure basic properties that change with temperature. In differential dielectric analysis (DDA) a property of the sample (dielectric constant) is measured while the temperature is changed. Further, in differential thermogravimetric analysis (DTGA), differential weight loss of a sample is monitored as the temperature is increased.
In 1968, Sullivan and Seidel reported a non-differential thermal technique which is now known as AC calorimetry. P. F. Sullivan, G. Seidel, "Steady-State, AC-Temperature Calorimetry, " Phys. Rev. 173(3), 679-685 (1968). This technique was later modified by Dixon et al. who, in 1982, reported a method called differential AC calorimetry. G. S. Dixon et al., "A Differential AC Calorimeter for Biophysical Studies," Anal. Biochem. 121(1), 55-61 (1982). Differential AC calorimetry, as described by Dixon et al., consists of heating or cooling the sample and reference at a linear rate with a sinusoidal oscillation superimposed on the linear heating or cooling program. Dixon et al. determined the heat capacity of the sample using the differential AC temperature response measured between the sample and reference.
U.S. Pat. No. 5,224,775, assigned to TA Instruments, Inc. (hereinafter "the '775 patent"), discloses the use of differential AC calorimetry in a method which deconvolutes the resulting differential signal as described by Dixon et al. The '775 patent discloses processing of the signal into "rapidly reversing" and "non-rapidly reversing" components. The thermodynamic significance of the "rapidly reversing" and "non-rapidly reversing" components is not apparent for time-dependent processes. For time-independent thermal events (equilibrium processes), only the "rapidly reversing" component may have thermodynamic significance. Since most thermal events of interest, such as the glass transition of a polymeric material, are time-dependent processes, there is an obvious need for a more comprehensive method of processing the differential signal.
The present invention provides a method and apparatus for processing the differential signal into real (inphase) and imaginary (quadrature) components which are related to the "energy storage" and "energy loss" portions of the thermal event being studied. The inphase and quadrature components provide physical and thermodynamic information for thermal events which are time-independent or time-dependent.